what physically determines the speed of a processor?
In: Technology
Design engineers call it “timing closure”. When you design a microprocessor, you have to connect the transistors with metal traces which act as the wires that go between them. For nearby transistors those traces are short and simple, but for ones further away they can be relatively long and go up and down through different layers, kind of like a printed circuit board.
To change a transistor from on to off, you’ve got to get the voltage on a trace to a certain level. Traces have both resistance and capacitance, which makes the signals a bit slow to change. The longer the trace, generally the slower it is to change its voltage.
This is all important because most circuits are “synchronous”…they are supposed to change voltages by a certain time. If they are late (or sometimes, too early) it will cause an error. So the circuits require careful timing analysis to make sure that all of the signals will reach their destination on time, even over variations in voltage and temperature.
Usually this initially results in lots of lots of failures. So signals have to be rerouted, drive strengths changed, and various other changes to make sure the circuit will work. Then they have to be analyzed again, and you find that those changes cause new problems. It’s an iterative process that takes a lot of work.
Additionally, you can run into other problems that need to be dealt with, such as cross-talk between nearby traces, hot spots, etc.
Once you’ve dealt with all of these issues and the signal timings are all good, you’ve “closed timing”. It’s a major milestone in microprocessor design.
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